1. Field of the Invention
This invention relates to a separable unitary composite fiber comprised of at least two different polymer components, one of which is soluble in a given solvent and the other of which is relatively insoluble in the given solvent, a plurality of segments of the relatively insoluble polymer component being isolated from each other by the intervening soluble polymer component in the transverse cross-section of the composite fiber. It relates further to a process for producing such a separable unitary composite fiber.
2. Description of the Prior Art
It is widely known that a separable unitary composite fiber comprised of a soluble polymer component and a relatively insoluble polymer component, and having a transverse cross-section such that a plurality of segments of the relatively insoluble polymer component are isolated from each other by the intervening soluble polymer component, can be divided into a plurality of fine, separate independent fibers by subjecting the unitary composite fiber to the action of a solvent capable of dissolving the soluble polymer component. By the term "relatively insoluble" used herein is meant that the polymer component is insoluble or only slightly soluble in a given solvent. The fine, separate independent fibers can be of an extremely small fineness, e.g. below one denier, and can constitute a woven or knitted fabric having a silk-like hand. The process of obtaining such a fabric from a separable unitary composite fiber resembles that of obtaining a silk fabric from a raw silk, i.e., a fibroin-sericin composite fiber, by dissolving out sericin from the composite fiber.
The following requisites are generally needed or desired for obtaining fine fibers from a separable unitary composite fiber.
(1) The difference in the rate of dissolution between the soluble polymer component and the relatively insoluble polymer component is large, and the rate of dissolution of the soluble polymer component is rapid.
(2) The apparatus and operation employed are not complicated, and the solvent used is not corrosive to the apparatus and is satisfactory from standpoints of cost and safety.
(3) The soluble polymer component is readily available and not costly.
(4) The soluble polymer component and the relatively insoluble polymer component are not liable to be undesirably separated from each other during the processes of spinning, drawing and weaving or knitting, conducted before the composite fiber is subjected to the action of a solvent.
It has been heretofore proposed in Japanese Patent Publication No. 42,847/1972 and Japanese Laid-open Patent Application No. 9,021/1973 that, for the purpose of producing polyester fibers characterized as possessing enhanced melting point, strength, modulus of elasticity and resistance to chemicals, and having good electrical properties, polystyrene and nylon-6 be used as the soluble polymer component of a separable unitary composite fiber. Polystyrene and nylon-6 are, however, not advantageous in that a solvent, which is expensive and not safe, such as a special organic solvent or a strong acid, is necessary for the dissolution of these soluble polymer components. Furthermore, nylon-6 is liable to be separated from the relatively insoluble polymer component prior to the step of dissolving nylon-6.
Another proposal has been made in which a composite fiber comprised of a relatively insoluble polyester and a soluble polyester readily compatible with the relatively insoluble polyester is subjected to the action of an aqueous alkaline solution. For example, U.S. Pat. No. 3,117,362 discloses a combination of polyethylene terephthalate as the relatively insoluble polyester with a copolymer comprised of polyethylene terephthalate and poly(ethylene oxide) glycol as the soluble polyester. However, the copolymer used as the soluble polyester must contain a large porportion of the units derived from poly(ethylene oxide) glycol, so that the copolymer will be readily dissolved in a given solvent. This copolymer which contains a large proportion of poly(ethylene oxide) glycol units is, however, poor in spinnability. Furthermore, Japanese Patent Publication Nos. 47,532/1972 and ibid. 47,533/1972 disclose a combination of polyethylene terephthalate as the relatively insoluble polyester with a polyethylene terephthalate composition, as the soluble polyester, having incorporated therein an additive such as a special polyalkylene glycol or an anionic surface active agent. The composite fibers disclosed in these two publications are still not advantageous in that the incorporated additive is difficult to distribute uniformly along the length of the fibers and, thus, the division of the composite fiber cannot be uniformly and smoothly effected. Furthermore, Japanese Patent Publication No. 33,415/1973 refers to the use, as the soluble polyester, of an alkali-soluble copolyester (containing polyethylene glycol or polypropylene glycol), a blend thereof, polyethylene sebacate, polyethylene adipate, a copolymer of di- or tri-ethylene glycol terephthalate or polyethylene terephthalate, or a copolyester containing polyethylene glycol/polypropylene glycol units. These soluble polyesters are generally difficult to melt-spin stably over a long period of time, or the resulting composite fibers cannot be uniformly and smoothly divided.
Still another proposal has been made in which a composite fiber comprised of a relatively insoluble polyamide and a soluble polyester is subjected to the action of a given solvent, thereby to dissolve out the soluble polyester from the composite fiber and to obtain polyamide fibers characterized as possessing enhanced melting point, strength, abrasion resistance and chemical resistance, and having good electrical properties. For example, Japanese Patent Publication No. 28,005/1973 discloses a separable unitary composite fiber comprised of a polyester component and a polyamide component. It is referred to in this reference that the polyester component may be removed from the composite fiber by using an aqueous alkaline solution. However, it is actually difficult to dissolve out the polyester component disclosed in this reference. Moreover, the polyester component and the polyamide component are liable to be separated from each other during the drawing process, which separation leads to fluff formation and yarn break during the weaving or knitting process.
U.S. Pat. No. 3,117,362 discloses a separable composite fiber comprised of four segments of polyhexamethylene adipamide separated by the intervening polyethylene terephthalate. This polyhexamethylene adipamide-polyethylene terephthalate composite fiber is also liable to be separated into the respective polymer components during the drawing process, and fluff formation and yarn break cannot be avoided.